End point detection in plasma etching reactions is known in the art. However, end point detection by monitoring gases with a gas analyzer in a non-plasma system has not been available till now, particularly in a flow-through or recirculation etch system. Though other types of end point detection methods have been used in etch systems (optical monitoring, electrical monitoring, etc.), such methods can be difficult to set up and inaccurate.
The present invention is in the area of the manufacture of MEMS (microelectromechanical systems) as well as semiconductor devices, or any other devices that require removal of a material in accordance with the present invention. In particular, this invention addresses gas-phase etching procedures, with particular emphasis on detection of the endpoint in an etching process. The invention is also directed to apparatus useful for etching and detecting the endpoint of the etching reaction. “MEMS”, “microelectromechanical” and “micromechanical” are used interchangeably throughout this application and each may or may not have an electrical component in addition to the microstructure component. The end point detected can be a point in an etch process where all of the material that is capable of reacting with the etchant gas has been removed and there is no more of the material remaining on the substrate or exposed to the etchant gas.
The use of etchants for removing sacrificial layers or regions in a multilayer structure without removal of an adjacent layer or region is a common step in the manufacture of semiconductor and MEMS devices. The MEMS devices of the present invention can be devices for inertial measurement, pressure sensing, thermal measurement, micro-fluidics, optics, and radio-frequency communications, with specific examples including optical switches, micromirror arrays for projection displays, accelerometers, variable capacitors and DC or RF switches. If a semiconductor device is etched, it can be any device that is made of or has thereon a material that is to be removed with a gas phase chemical etchant.
The success of an etch step in the manufacture of microstructures is improved not only due to the selectivity of the etchant, but also due to the ability to accurately determine the endpoint of the etching process. Isotropic etching is of particular interest in processes where the purpose of the etch is to remove a sacrificial layer that is intervening between functional layers or between a functional layer and a substrate. Gas phase etchants, particularly in the absence of plasma, are desirable for isotropically removing a sacrificial layer.
Of potential relevance to certain embodiments of this invention is the prior art relating to particular etchant gases. Prominent among the etchants that are used for the removal of sacrificial layers or regions in both isotropic and anisotropic etching procedures are noble gas fluorides and halogen fluorides. These materials, used in the gas phase, selectively etch silicon relative to other materials such as silicon-containing compounds, non-silicon elements, and compounds of non-silicon elements. Descriptions of how these materials are used in etching procedures appear in co-pending U.S. patent application Ser. No. 09/427,841 and Ser. No. 09/649,569 to Patel et al. and in portions of the present specification that follow.
The method of the present invention is useful for detecting an endpoint in methods for producing deflectable MEMS elements (deflectable by electrostatic or other means) which, if coated (before or after gas phase processing) with a reflective layer, can act as an actuatable micromirror. Arrays of such micromirrors can be provided for direct view or projection display systems (e.g. projection television or computer monitors). If the micromirrors are provided alone or in an array and of a size of preferably 100 micrometers or more (preferably 500 micrometers or more), the mirror can be useful for steering light beams, such as in an optical switch. The present invention is also adaptable to detecting an endpoint in methods for etching microfabricated devices other than MEMS devices (e.g. semiconductor based devices, carbon nanotubes on glass, etc.)